The present invention relates to techniques for monitoring a process, and more particularly to a method and apparatus for monitoring a process by employing principal component analysis.
Within the semiconductor industry, an ever present need exists for improved process repeatability and control. For example, during the formation of a typical metal-layer-to-metal-layer interconnect, a dielectric layer is deposited over a first metal layer, a via hole is etched in the dielectric layer to expose the first metal layer, the via hole is filled with a metal plug and a second metal layer is deposited over the metal plug (e.g., forming an interconnect between the first and the second metal layers). To ensure the interconnect has low contact resistance, all dielectric material within the via hole must be etched from the top surface of the first metal layer prior to formation of the metal plug thereon; otherwise, residual high-resistivity dielectric material within the via hole significantly degrades the contact resistance of the interconnect. Similar process control is required during the etching of metal layers (e.g., Al, Cu, Pt, etc.), polysilicon layers and the like.
Conventional monitoring techniques provide only a rough estimate of when a material layer has been completely etched (i.e., endpoint). Accordingly, to accommodate varying thicknesses of material layers (e.g., device variations) or varying etch rates of material layers (e.g., process/process chamber variations), an etch process may be continued for a time greater than a predicted time for etching the material layer (i.e., for an over-etch time). Etching for an over-etch time ensures that all material to be removed is removed despite device variations and process/chamber variations that can vary etch time.
While over-etch times ensure complete etching, over-etching increases the time required to process each semiconductor wafer and thus decreases wafer throughput. Further, the drive for higher performance integrated circuits requires each generation of semiconductor devices to have finer dimensional tolerances, rendering over-etching increasingly undesirable. The smaller open areas required for reduced dimension device structures also reduce the intensity of commonly monitored electromagnetic emissions (e.g., reaction product emissions) so as to render monitoring techniques employing narrow band intensity measurements increasingly difficult and inaccurate. Accordingly, a need exists for improved techniques for monitoring semiconductor manufacturing processes such as etch processes, chamber cleaning processes, deposition processes and the like.
The present inventors have discovered that by measuring correlated attributes of a process (e.g., a plurality of electromagnetic emissions, and/or process temperature, process pressure, RF power, etc.), and by employing principal component analysis to analyze the correlated attributes, process state, process event and, if applicable, chamber state information may be easily and accurately obtained for the process. Exemplary process state information that may be obtained includes RF power, plasma reaction chemistry, etc.; exemplary process event information that may be obtained includes whether a particular material has been etched through or away (i.e., breakthrough), whether a desired process is complete (e.g., etching or deposition), when a wafer is improperly held (i.e., improper xe2x80x9cchuckingxe2x80x9d), etc.; and, if applicable, exemplary chamber state information that may be obtained includes whether a chamber contains a fault, whether a chamber""s operation is similar to its previous operation or to another chamber""s operation (i.e., chamber matching), etc.
In accordance with the invention, correlated attributes are measured for the process to be monitored (i.e., the production process), and principal component analysis is performed on the measured correlated attributes so as to generate at least one production principal component. The at least one production principal component then is compared to a principal component associated with a calibration process (i.e., a calibration principal component).
The calibration principal component is obtained by measuring correlated attributes of a calibration process (e.g., preferably the same process as the production process, but typically for non-production purposes), and by performing principal component analysis on the measured correlated attributes so as to generate at least one principal component. A principal component having a feature indicative of at least one of a desired process state, process event and chamber state then is identified and is designated as the calibration principal component. Preferably the at least one production principal component is compared to the calibration principal component by computing the inner product of the calibration and production principal components. The calibration and production principal components also may be compared by employing other techniques such as the xe2x80x9ccoherencexe2x80x9d function found in the mathematics software package MATLAB(trademark) marketed by Mathworks, Inc. or by computing the scalar magnitude or xe2x80x9cnormxe2x80x9d of the difference between the calibration and production principal components.
By thus comparing calibration and production principal components, process event, process state and chamber state information may be obtained rapidly (e.g., in real time) and with a high degree of accuracy. Processes thereby may be monitored and processing parameters/conditions adjusted in real time, over-processing times such as over-etch times avoided and process yield and throughput significantly increased.